Electron-beam lithography is used to transfer, to a substrate, with a high resolution, geometric patterns forming a layout. An electron beam is used to expose a resist deposited on the substrate according to the layout to be transferred. The exposed resist undergoes a chemical transformation that allows it to be selectively removed, uncovering certain regions of the substrate that may then be etched or undergo a deposition or an ion implantation (alternatively, it is the unexposed resist that is selectively removed).
The main application of electron-beam lithography is the manufacture of the photolithography masks that are used to produce integrated circuits. Other applications are the direct manufacture of integrated circuits, of photonic or electronic components, and of nanostructures.
The resist may be exposed point by point, by means of a very narrow electron beam, but this takes a lot of time. For this reason, recourse is generally made to another technique, called the variable-shaped-beam (VSB) technique, the principle of which is illustrated in FIGS. 1A, 1B and 2. As may be seen in FIGS. 1A and 1B, a relatively wide electron beam FE, generated by an electron source SE, passes through two successive apertures O1, O2. The shape of the beam after it has passed through the apertures depends on the shape of the latter, which is variable. Thus an elementary pattern (a “shot” is spoken of in the literature) ME, ME′, the shape of which approximately corresponds to that of the beam, is obtained, which is transferred, in a single exposure, to the substrate. In the case of FIG. 1A, the elementary pattern ME is square or rectangular; in that of FIG. 1B, the elementary pattern ME′ is triangular.
In fact, the shape and dimensions of the elementary pattern actually transferred to the substrate does not correspond exactly to those of the beam, and also depend on nearby patterns (for this reason “proximity effects” are spoken of). This is mainly due to scattering of the electrons in the resist and to backscattering thereof by the substrate.
To determine the pattern actually transferred to the substrate, the following are applied to a “nominal” pattern:
a physical model, which represents the spread of the electrons in the resist, generally by means of a point spread function (PSF), and
a model of the resist—generally a simple threshold-based model: the resist is considered to be exposed if the electron dose that it receives exceeds a threshold.
As is known per se, this allows the corrections that must be made to the nominal pattern to ensure the transferred pattern is as close as possible to that desired to be determined. “Data preparation” is spoken of because this operation results in the creation of a data file that is delivered to the VSB machine to control the execution of the lithography process in order to obtain the transfer of the sought-after pattern.
Typically, VSB machines allow rectangular or square, or even right-isosceles-triangle-shaped elementary patterns having an orientation of 0°, ±45° or 90° with respect to a reference direction to be obtained. These various elementary shapes are illustrated in FIG. 2. It will easily be understood that they allow, simply and rapidly, certain patterns, for example lines with these four predefined orientations, to be produced. It may for example be seen in FIGS. 3A and 3B that lines that are horizontal (90° with respect to the vertical reference direction) or at 45° may be obtained from a limited number of elementary patterns having substantially the same critical dimension as the line itself, said dimension being corrected to take into account proximity effects (the “critical dimension” is the smallest dimension of a pattern: the width of a line, the side length of a square, etc.). In contrast, a line having a different orientation must be decomposed into a larger number of elementary patterns that are substantially smaller than its critical dimension (width)—this is illustrated in FIG. 3C.
Now, it is known that when elementary patterns of very small dimensions (typically 100 nm or less) are produced by variable-shaped-beam electron-beam lithography, the dimensions of the patterns actually transferred to the substrate differ from those expected from physical model and model of the resist alone. For example, FIG. 4 shows the critical dimension CDm (width) measured for a straight line of nominal critical dimension CD0 equal to 120 nm, obtained by VSB electron-beam lithography, as a function of its orientation. It may be verified that the measured critical dimension CDm differs little from its nominal value, given by a physical model (horizontal line), for an orientation of 0°, 45°, 90°, 135°, because in these cases it is not necessary to use elementary patterns that are smaller than the critical dimension. In contrast, for intermediate orientations, the measured critical dimension of the line may exceed 130 nm, i.e. an error of about 10%.
This effect is known to the scientific literature:    H. C. Pfeiffer et al. “Recent Advances in Electron-Beam Lithography for the High-Volume Production of VLSI devices”, IEEE transaction on electron devices, Vol. ED-26 4, 663 (1979);    S. Nishimura et al., “Evaluation of Shaping Gain Adjustment Accuracy Using Atomic Force Microscope in Variably Shaped Electron-Beam Writing Systems”, J. Appl. Phys. 36, 7517 (1997);    J. Choi et al., “Requirements of e-beam size and position accuracy for photomask of sub-32 nm HP device”, SPIE Vol. 7748, 774819-1 (2010);    S. Park et al., “Requirements of the e-beam shot quality for mask patterning of the sub-1X device”, SPIE Vol. 9777, 977716-1 (2016).
However, there is no method allowing the errors associated with the use of “small” elementary patterns, i.e. elementary patterns smaller than the critical dimension of the pattern be transferred, to be systematically and simply corrected.